This invention relates to an automatic focusing apparatus including an optical system, more specifically to an apparatus best suited for automatic focusing of microscopes.
In conducting a microscopic inspection of a photo mask plate of an IC or LSI, a focal deviation may be caused when a stage is moved, which is attributable to the uneven planeness of the photo mask plate.
As automatic focusing apparatus for automatically correcting such focal deviation, there are conventionally known air-micro type apparatus which make use of air pressure.
According to one such air-micro type automatic focusing apparatus, the body tube of an objective of a microscope is transformed and given the function of an air nozzle, air is blown at a predetermined pressure against a sample placed on the stage, and the back pressure of the air is always kept constant, thereby automatically correcting the focal deviation. If it is assumed that the focus is properly adjusted with the back pressure at a value when the sample is located at a predetermined distance (e.g., 100.mu.) from the air nozzle, the back pressure rises or falls as the sample approaches or goes away from the air nozzle with the shift of the stage (i.e., the focus is deviated), respectively. It is intended that the change in the back pressure be detected with a high-sensitivity pressure sensor, and that the objective be vertically shifted so as continually to keep the output of the pressure sensor constant, thereby maintaining the most correct focus.
The vertical shifting of the objective is performed by means of a motor system which operates in response to the output signal of the high-sensitivity pressure sensor.
These conventional automatic focusing apparatus, however, have various shortcomings as follows. That is, in the air-micro type automatic focusing apparatus, the body tube of the objective of the microscope is transformed into the air nozzle, so that there is required a special tube for the objective which must be provided with an air inlet portion for the air supply. Since the air inlet portion need be connected with an air hose, the mechanism surrounding the objective becomes large-sized and sophisticated, increasing the cost of production. In addition, replacement of the objective for the change of magnification requires operations to remove the air hose from the body tube and to attach it to a new one after the replacement, leaving a question with respect to the operation efficiency. In particular, this is not very convenient for example in the photo mask inspection which involves replacement of the objective for the change of magnification. Since the air nozzle must be supplied with compressed air, it is necessary to provide an air pump connected with the air hose. The compressed air is to be ejected from the air nozzle under a fixed pressure, and the pressure sensor to measure the back pressure is required to exhibit high sensitivity.
Moreover, in the air-micro type apparatus, focusing is achieved by vertically shifting the objective as aforesaid, part of the optical system of the microscope is to be shifted. Such partial shifting may, however, cause some deviation of the optical axis of the optical system of the microscope.
Furthermore, the air-micro type automatic focusing apparatus detects a change in the back pressure (i.e., focal deviation) by means of the pressure sensor, vertically shifting the objective in accordance with the result of such detection. Therefore, the shift mechanism must be coupled to the support member of the objective. Accordingly, the mechanism surrounding the support member of the objective becomes highly complicated. Moreover, the shifting is performed by using a motor, which must be mounted on the microscope, and thus the conventional construction of the microscope needs to be changed substantially. Supposed to measure the back pressure, the conventional focusing apparatus of the air-micro type have some other defects; extreme reduction in the detection sensitivity accompanying an increase in shift distance and a limited dynamic range.
Thus, the conventional automatic focusing apparatus are still subject to various shortcomings as regards the mechanism, operation efficiency, or dynamic range.